1. Field of the Invention
The present invention relates to power control circuitry, circuitry for analysing an on-chip switched power rail, and a method of controlling connection of a power source to an on-chip switched power rail.
2. Description of the Prior Art
It is known to provide circuits that include power rails connected via switch blocks to switched power rails. Circuit blocks can then be arranged to draw their power from the switched power rails. The switch blocks may be provided as header switch blocks for connecting a supply voltage rail to a switched supply voltage rail, or may be footer switch blocks for connecting a ground voltage rail with a switched ground voltage rail. Indeed, some circuits may include both header switch blocks and footer switch blocks. The switch blocks are typically constructed using high threshold transistors, and can be used to isolate the switched power rail from the main power rail and accordingly isolate the associated circuit blocks from the power source. This is useful in reducing power consumption within the circuits, for example by reducing the static leakage current therethrough.
The operation of the various switch blocks is typically controlled by a switch controller, and the switch controller will typically control the operation of the switch block having regards to expected characteristics of the switch block, such as expected turn on time, turn off time, etc. However, the actual characteristics of the switch block will vary depending on process variations in manufacture, local temperature, variations in supply voltage, etc. If the actual characteristics of the switch block could be determined in situ, it would enable the operation of the switch block to be managed more efficiently, potentially allowing quicker turn on and turn off characteristics, less power consumption, etc.
However, to determine the actual characteristics of the switch block, it would be useful to observe one or more properties of the analogue voltage present on the switched power rail, but it is difficult to make such analogue voltage measurements in an unintrusive manner. Analogue voltage measurements typically require exposing the switched power rail off chip, which then adds capacitive load and itself affects the turn on characteristic. Another possibility would be to seek to develop mixed-signal analogue-to-digital converters for deployment on-chip, but such direct access techniques may tend to compromise the measurement more than an indirect approach.
In addition to using the switch block to connect the switched power rail to the power source so as to enable normal operation of the associated circuit block, or to decouple the switched power rail from the power source when the circuit block is turned off so as to reduce power consumption, the switch controller may also control the switch block in a more complex manner to achieve other modes of operation. For example, commonly assigned co-pending U.S. patent application Ser. No. 11/797,497, the entire contents of which are hereby incorporated by reference, describes the use of a state retention mode for a circuit block, where the switch controller modulates conduction through the switch block to maintain the switched power rail at an intermediate voltage level. To improve the efficiency of such mechanisms, it would also be beneficial to readily determine information about the analogue voltage on the switched power rail at particular points in time, as that would assist in performing the required modulation.
Accordingly, it would be desirable to provide a simple and effective mechanism for observing the switched power rail so as to improve power control using a switch block coupled between a power source and a switched power rail.